Natural electromagnetic fields are electromagnetic fields originating mainly from lightning discharges in the Earth-ionosphere cavity. At some distance from their source, these fields propagate as a plane wave with respect to the horizontally directed vector of the magnetic field. There are two components of the field each having different properties.
The first component of natural electromagnetic fields is caused by global thunder activity. The signal produced by electromagnetic fields of this source are relatively stable. A record of this signal received in a manner that is known using a coil as a sensor appears to be random noise. It has almost a homogeneous spectral density in the range between 8 and 500 Hz. Horizontal components of this signal have spectral density of 0.05-0.15 nV/(Hz 1/2×m2). The signal can change gradually several times a day depending on such factors as time of day, time of year and geographical location depending on changes in the conditions of the conductive layers in the ionosphere. Generally the worst time for measurements is winter midday because of the relatively low ambient electromagnetic fields.
The second component of natural electromagnetic fields is caused by local thunderstorms generally happening within a radius 700-1000 km. This signal is more intermittent and appears on a known sensor as separate pulses of oscillations of 1-5 ms each. The spectrum of the field is most intensive in the 2000 to 20000 Hz range. Its intensity can change significantly in a matter of hours.
The natural electromagnetic field penetrates through the ground to certain depth h which depends on the electric resistivity of the ground Ro and frequency of the field f. In the presence of a homogeneous non-magnetic conductive halfspace, the depth of field penetration h is determined in accordance with the following approximate formula:h=356(Ro/f)1/2 [m].So, if we assume Ro=500 Ohm*m, f=500 Hz, then depth of field penetration h will be equal to approximately 356 meters.
Prior art solutions have been devised to extrapolate geological survey data from data collected regarding natural electromagnetic fields. For example, U.S. Pat. No. 3,149,278 ('278) issued to McPhar Geophysics Limited, invented by W. O. Cartier et al. The referenced prior art solution was based on measuring the tilt angles of the magnetic field using two or three electromagnetic coil systems mounted inside a bird towed behind an aircraft.
As, discussed in U.S. Pat. No. '278 naturally occurring electromagnetic fields exist having electric and magnetic field components in the audio frequency range. The prior art solutions discussed could only be used at defined times of the year (usually summer where there is a greater incidence of thunderstorm activity) because the prior solution could not function in low ambient electromagnetic fields. Other errors can be caused by instability of the airborne electromagnetic coil assembly in horizontal attitude during flight that causes angular rotations thereof producing errors in measuring the tilt angles. Furthermore, a geographical relief also distorts horizontal magnetic fields and produces false anomalies of tilt angles even if there are very homogeneous rocks beneath the Earth surface.
Furthermore, the prior art solutions do not permit deriving survey data from electromagnetic field measurements in relation to relatively low ambient electromagnetic fields, including in low ambient electromagnetic fields in the order of 0.05-0.15 nV/(Hz1/2×m2). This is particularly the case because having regard to such low ambient electromagnetic fields prior art solutions were unable to differentiate electromagnetic field measurements from noise.
There are a number of sources of noise that it is preferable to discount from electromagnetic field measurements to provide useful survey data.
The first source of noise is microphonic noise caused by the mechanical vibration of a sensor in the magnetic field of the Earth. In the case of the use of airborne electromagnetic sensors the mechanical vibrations of engines and/or propellers, as well as air turbulence causes this sensor vibration.
In addition, the aircraft electrical equipment and moving magnetic parts produces further electromagnetic noise.
In addition there is an internal thermal noise from sensors and input circuitry.
All of the above factors have a distorting effect on survey data extrapolated from measurement of natural electromagnetic fields. The above factors are collectively referred to in this disclosure as “Distorting Factors”.
There is a need therefore for an apparatus for measuring natural electromagnetic field data that reduces the impact of such Distorting Factors. There is a further need for a system, computer product and method for extrapolating geological mapping data from natural electromagnetic field data in an efficient and cost effective manner.
In the drawings, embodiments of the invention are illustrated by way of example. It is to be expressly understood that the description and drawings are only for the purpose of illustration and as an aid to understanding, and are not intended as a definition of the limits of the invention.